Semiconductor bistable switches may be used in high current applications such as, for example, power supplies and/or other electrical power conversion applications. For example, thyristors may be solid-state semiconductor devices with four layers of alternating N and P-type material. Thyristors may act as bistable switches, conducting when they are forward biased and their gate receives a current pulse, and continuing to conduct as long as they are forward biased. Thyristors may generally have three states: 1) reverse blocking when voltage is applied in a direction that would be blocked by a diode; 2) forward blocking when voltage is applied in a direction that would cause a diode to conduct, but the thyristor has not been triggered into conduction; and 3) forward conducting mode when the voltage is applied in a direction that would cause a diode to conduct and a triggering current has been received at the gate. Once a thyristor is in the forward conducting mode, the device remains latched in the on-state as long as the anode remains positively biased relative to the cathode (i.e., until the anode current falls below a holding current.)
A thyristor in forward conducting mode may be switched off when an external circuit causes the anode to become negatively biased. In some applications, this may be done by switching a second thyristor to discharge a capacitor into the cathode of the first thyristor. This technique may be referred to as forced commutation. For example, a conventional gate turn off thyristor (GTO) may be turned off by either shorting or applying reverse bias to the gate to cathode junction. A large amount of current may be typically necessary to do this.
Brief reference is made to FIGS. 1a, and 1b, which are a symbol and equivalent circuit schematic diagram, respectively, of a conventional gate turn off thyristor. Although normal thyristors may not be fully controllable switches (a “fully controllable switch” can be turned on and off at will), a GTO 10 can be turned-on by a gate signal and can also be turned-off by a gate signal of negative polarity. Like a normal thyristor, GTO turn on may be accomplished by a current pulse between the gate terminal 16 and the cathode terminal 14. As the gate-cathode may behave like a PN junction, there may be some relatively small voltage between the terminals. Accordingly, the turn on phenomenon in a GTO may not be as reliable as a conventional thyristor and thus a small positive gate current may be provided even after turn on to improve reliability.
Turn off in a GTO may accomplished by applying a negative voltage pulse between the gate terminal 16 and the cathode terminal 14. Some of the forward current (about one-third to one-fifth) may be drained and used to induce a cathode-gate voltage, which in turn induces the forward current to fall and the GTO to switch off (transitioning to a blocking state.)
In addition to requiring significant currents to turn off, GTO's may suffer from long switch off times. For example, after the forward current falls, there may be a long tail time where residual current continues to flow until all remaining charge from the device is taken away.
Furthermore, a conventional GTO structure may have a maximum limit on controllable current. Additionally, in conventional thyristors, the load current can reach a point where it cannot be turned off by reverse biasing the gate. This limitation may be on the order of twice rated operating current.
Other technology may use a high power gate driver connected to turn off the thyristor by reverse biasing the gate to cathode junction. Cascode techniques have been done in the past, but cascode devices (aka Emitter-Turn-Off (ETO) thyristor) have been realized with discrete GTO thyristor in series with a discrete MOSFET. Such devices may have additional conduction losses with the discrete MOSFET and difficulty commutating the devices at high load currents and high dV/dt due to stray inductances between the MOSFET and the GTO.
GTO's with integrated turn-off switches have been proposed in the past, namely the MOS controlled thyristor (MCT). Such structures may use integrated N and P channel MOSFETs to control the thyristor structure. The PMOS transistor may be used to turn the device on and the NMOST transistor may be used to turn off the device. Turn off may be accomplished by shorting out the base to emitter junction of the NPN portion of the thyristor structure. A disadvantage of this device may be its maximum controllable current may be limited due to a finite resistance of the NMOS transistor.